I spent a few years of my life backpacking in a few countries and learned how how to manage my stuff in order to live out of a 35 litre backpack. I had to be mindful of what I took with me with such limited space.
I've since carried that approach to my life at home and tend to be proactive in dealing with what I buy and own. One way I try to be proactive is to make a list of stuff I want or think I need instead of buying it as soon as possible. Most times something sits on the list for a long time and I'll just remove it. It's a good indicator that I don't need to buy it and it will only cause more clutter. This is sort of something I made up over time to work around my adhd impulsiveness so it may or may not help others.